1. Field of the Invention
The present invention relates to equipment for cleaning steam generators and in particular to a high pressure fluid lance for cleaning sludge off the steam generator tubes near the lower tubesheet.
2. General Background
In nuclear power stations, steam generators such as recirculating steam generators (RSG's) and once-through steam generators (OTSG's) are used for heat exchange purposes in the generation of steam for driving turbines. Primary fluid which is heated by the core of the nuclear reactor is led through a collection or bundle of tubes in the steam generator. A secondary fluid, generally water, which is fed into the space around the tubes receives heat from the tubes and is converted to steam for driving the turbines. After cooling and condensation, the secondary fluid is directed back into the space around the tubes to provide a continuous steam generation cycle. Due to the constant high temperature and severe operating environment, a sludge mainly comprised of an iron oxide such as magnetite builds up on the lower outer portion of the tubes and on the tubesheet which supports the tube handle. As the sludge buildup on the tube bundle reduces the heat transfer efficiency of the tubes and can cause corrosion, it is preferable that the tubes be periodically cleaned to remove the sludge. Cleaning methods which applicants are aware of include the following.
U.S. Pat. No. 4,566,406 entitled "Sludge Removing Apparatus For A Steam Generator" discloses a manifold which is rigidly attached to the tubesheet and remains in place during conventional operation of the steam generator. A plurality of nozzles on the manifold emit streams of water to break up sludge on the upper surface of the tubesheet and openings provided in the walls of the steam generator are used to remove the slurry.
U.S. Pat. No. 4,422,882 entitled "Pulsed Liquid Jet-Type Cleaning Of Highly Heated Surfaces" discloses a method and apparatus for removing deposits from the fire sides of the tubes of boilers while steaming by employing a sootblower to project a moving pulsed jet of liquid against the deposits. The peak impact pressure of the jet is increased by pulsing means such as a fluidic or rotary type.
U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" discloses an arrangement of headers at the elevation of the sludge to be removed for establishing a circumferential fluid stream at that elevation. A fluid lance moved along the line between the headers emits a fluid jet perpendicular to the line of movement of the fluid lance. The fluid lance may also be rotated as it is moved. This system is indicated for use in RSG's.
In OTSG's, a cleaning method also used is that of feeding water in the generator at the top and suctioning out at the bottom in an attempt to loosen and remove the sludge.
A problem encountered in many OTSG's is that access ports, known as handholes in the art, to the tube bundle and tubesheet are normally provided at a level above the tubesheet. This makes it difficult or impossible with existing equipment to reach and clean the tube bundle at the tubesheets in steam generators which have not been designed or retro fitted with special cleaning equipment.